The owner of the Exton Square Mall and Parking Garage was constructing two parking garages and a new second-story mall level. New building columns for the new mall level were located inside many retail stores. The challenge for Nicholson was to complete all pile installation that was necessary inside the mall at night. It was essential that business during the day occurred without disruption. Drilling conditions inside store areas were difficult to access, with headroom in some areas less than 12 feet. Merchandise had to be moved and protected. Business in the mall carried on “as usual” without much evidence of the night-time construction activity.
In addition to the physical constraints of the project site, the underlying bedrock consisted of karstic limestone with voids and clayseams presented challenges. The top of competent bedrock ranged from 20 to 150 feet below the existing ground surface. Despite these challenges, Nicholson installed 760 micropiles in approximately 6 months. The maximum design working load of each pile was 300 kips in compression. The piles were installed outside of the mall with large-track mounted drill rigs, while the inside piles were installed with electric powered mini drill rigs. All piles were installed using the rotary eccentric percussive drilling technique to advance the casing through the voids and clayseams in the karstic formation and into the underlying competent bedrock.
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